1. Field of the Invention
This invention concerns the direct conversion of petroleum residua in the presence of hydrogen. More particularly, the present invention relates to a process for the catalytic upgrading of residual oils in the presence of hydrogen to obtain lower boiling components.
2. Description of the Prior Art
Current practice for upgrading high molecular weight, hydrogen-deficient, high-impurity refinery stocks generally involves either hydrotreating followed by catalytic cracking, or hydrocracking, both of which involve the use of hydrogen in high-pressure process units. The need to "hydrogen treat" residual petroleum fractions produced by atmospheric and vacuum distillation of crude petroleum is due to the high contaminant content of such fractions. During the distillation of crude oil most of the contaminants present in the crude remain in the residual fraction. Such contaminants include metals and non-metals.
Principal metal contaminants are nickel and vanadium, with iron and small amounts of copper sometimes present. Additionally, trace amounts of zinc and sodium are found in some feedstocks. The high metals content of the residual fractions generally preclude their effective utilization as charge stocks for subsequent catalytic processing, e.g., catalytic cracking. The metal contaminants tend to deposit on the special catalysts for these processes and cause the formation of inordinate amounts of coke, dry gas and hydrogen.
Non-metallic contaminants such as sulfur, nitrogen and CCR generally preclude the direct use of residual fractions as fuels due to environmental concerns, i.e. pollution.
At present, catalytic cracking is generally conducted utilizing hydrocarbon chargestocks lighter than residual fractions which generally have an API gravity less than 20. Typical cracking chargestocks are coker and/or crude unit gas oils, vacuum tower overhead, etc., the feedstock having an API gravity from about 15 to about 45. Since these cracking chargestocks are distillates, they do not contain significant proportion of the large molecules in which the metals are concentrated. Such cracking is commonly carried out in a reactor operated at a temperature of about 430.degree. C. (800.degree. F.) to 820.degree. C. (1500.degree. F.), a pressure of about 100 kPa (1 atm) to 510 kPa (5 atm), and a space velocity of about 1 to 1000 WHSV.
The amount of metals present in a given hydrocarbon stream is often expressed as a chargestock's "metals factor." This factor is equal to the sum of the metals concentrations, in parts per million, of iron and vanadium plus ten times the concentration of nickel and copper in parts per million, and is expressed in equation form as follows: EQU F.sub.m =Fe+V+10 (Ni+Cu)
Conventionally, a chargestock having a metals factor of 2.5 or less is considered particularly suitable for catalytic cracking. Nonetheless, streams with a metals factor of 2.5 to 25, or even 2.5 to 50, may be used to blend with, or as all of the feedstock to a catalytic cracker, since chargestocks with metals factors greater than 2.5 in some circumstances may be used to advantage, for instance with the new fluid cracking techniques.
In any case, the residual fractions of typical crudes will require treatment to reduce the metals factor. As an example, a typical Kuwait crude, considered of average metals content, has a metals factor of about 75 to about 100. As almost all of the metals are combined with the residual fraction of a crude stock, it is clear that at least about 80% of the metals and preferably at least 90% needs to be removed to produce fractions (having a metals factor of about 2.5 to 50) suitable for cracking chargestocks.
Metals and sulfur contaminants present similar problems with regard to hydrocracking operations which are typically carried out on chargestocks even lighter than those charged to a cracking unit. Hydrocracking catalyst is so sensitive to metals poisoning that a preliminary or first stage is often utilized for trace metals removal. Typical hydrocracking reactor conditions consist of a temperature of 205.degree. C. (400.degree. F.) to 540.degree. C. (1,000.degree. F.) and a pressure of 790 kPa gage (100 psig) to 24,235 kPa gage (3,500 psig).
One method for catalytic cracking of residual oils is presented in U.S. Pat. No. 3,886,060. In this method, residual oil serves as a quench medium for limiting the conversion of a recycle oil product thereof in a riser conversion zone.
Catalytic conversion of metal containing residual feedstocks is described in various patents. A method for catalytically converting residual hydrocarbons containing greater than 1 ppm of metal contaminants in the presence of a low molecular weight carbon-hydrogen fragment contributing material and an acid zeolite catalyst is disclosed in U.S. Pat. No. 4,002,557. In U.S. Pat. No. 4,162,213, a process is described for catalytically cracking a metal-contaminated residual feedstock in the absence of hydrogen in a fluid cracking process wherein the regenerated catalyst has less than about 0.05 wt. % residual carbon.